''Young people kill themselves because of your attitudes!'' came the upset reply.

Inside, the public galleries were warned by the speakers of both houses against outbreaks of cheering and disruption among women's rights, gay rights, and traditional-values groups.

Advertisement

Politicians have quietly grumbled that the collision of two conscience votes is exhausting, it's not only the soul-searching that must accompany each decision, but the huge email avalanche that has descended on parliamentary in-boxes, and the phone calls, as interest groups weigh in.

Labor MP Peter Primrose told the Legislative Council he would support the same-sex marriage bill because he wasn't threatened by gay marriage, but he did feel threatened by ''those who email me and tell me that I am sick, wicked and bad, and that my offspring and I will suffer and burn for eternity if I support this bill''.

Number crunchers are trying to work out what effect each bill will have on the vote of the other, if any.

Political theorist with the University of NSW, Peter Balint, studied conscience votes in the ACT Parliament to determine what factors held most sway in a so-called ''free vote''. ''Only one of them was significant - party,'' he said.

His research found that despite the optics of a conscience vote, where a leader allows MPs to throw aside party loyalties because of the social and moral issues at stake, politicians largely continue to vote along party lines and follow the ''thought leader''.

''If they don't know anything about the issue, MPs will follow the lead of a key figure in the party. We saw it under John Howard with marriage equality. MPs will vote with one eye on their career. We call it the informal whip,'' Balint says.

Beyond this trend, the other factor that came into play was personal experience. Gender, religion and the electorate played a much lesser role, his study showed.

''Gender only matters on gender issues,'' Balint says. ''It matters on abortion, where women are more socially liberal. But that is because of personal experience - it is the lived experience as a woman that will determine your views.''

So what have NSW parliamentary leaders said on the two looming conscience votes?

Barry O'Farrell has previously said he doesn't flag voting intentions in a conscience vote because he doesn't want to influence MPs. But as the same-sex marriage debate opened in Parliament, the front page of the gay and lesbian newspaper Sydney Star Observer screamed: ''Jilted Barry says no.''

O'Farrell had written an opinion piece revealing his intention to vote against the bill. Although he supported same-sex marriage, O'Farrell said he wanted the Commonwealth to legislate at a national level, not the states.

This position was taken by several Liberal and National MPs in Parliament that morning, including the Nationals chairman of the inquiry that had concluded NSW had the constitutional right to legislate on same-sex marriage. Liberal Scott McDonald, who has family members in same-sex relationships, called on Prime Minister Tony Abbott to reconsider the issue.

Labor leader John Robertson supports same-sex marriage. On Zoe's Law, Robertson has already spoken in Parliament against the bill.

Nationals leader Andrew Stoner (who opposes same-sex marriage) has said he will listen to the debate on Zoe's Law before making a decision.

O'Farrell told radio station 2UE that opponents of Zoe's Law were legitimately expressing their views, but he believed ''there is nothing in this bill'' that deals with abortion.

National and Liberal MPs are lining up to speak both for and against it.